The present invention relates to a method for stripping organic coatings from coated objects. More particularly, the present invention concerns a method for stripping a coating obtained from compositions based on organic resins and/or prepared with organic vehicles, such as paint, shellac, varnish, lacquer and the like, as well as various oils and asphalts. The method of the invention is especially useful for removing such coatings from objects having irregular surfaces and from large surfaces, including vertical and inclined surfaces in the interior of large constructions, such as storage bins and tanks on land and the holds and ballast tanks of ships.
Commonly, paint is stripped from painted objects by application of an organic or inorganic solvent or mixture theeof. As discussed in Kirk-Othmer's ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 14, pp. 485-493, 2nd Edition, John Wiley and Sons, 1967, organic paint removers generally fall into three classes: compositions based on chlorinated hydrocarbon solvents, compositions consisting of mixtures of other solvents and removers based on aqueous solutions or dispersions of phenols and/or organic acids and other compounds. Inorganic strippers, such as an aqueous solution of caustic soda and in some cases, mineral acids are also used, particularly for industrial applications.
Among the chlorinated hydrocarbon solvents, methylene chloride (dichloromethane) has been found to be particularly effective and formulations of methylene chloride suitable for application by dip, brush, spray and delivery from aerosol cans are known. Such compositions also usually contain additives including thickeners, evaporation retarders and detergents.
Organic solvent formulations for stripping paint and other coatings may be of the "scrape off" type or "flush off" type. Generally, the stripping composition is applied to the coated object by one of the foregoing methods and allowed to stand for some time, after which, the coating which has become swollen and/or softened is removed from the surface, by scraping, in the case of "scrape off" formulations or by flushing with water and/or by wiping with a damp rag in the case of "flush off" formulations.
The foregoing methods are relatively expensive, since the organic solvent, except in the case of application by immersion, is not in a form which can be recovered practically. Moreover, all of the foregoing procedures are generally impractical and prohibitively expensive where large surfaces are involved. In addition, extreme safety measures would be required to effectively treat large surfaces by any of the foregoing methods; the measures necessary to protect personnel from stripping chemicals, many of which are exceedingly toxic, essentially prohibit their use for stripping large objects. Another important problem with the foregoing procedures is the difficulty of completely removing the additives from the stripped surface, particularly the waxes used as evaporation retarders in formulations of organic chemical stripping compositions; any residual wax interferes with the adhesion of subsequent coatings to the surfaces.
Processes have also been described in U.S. Pat. Nos. 2,689,198 to Judd; 3,794,524 to Nogueira et al and 3,832,235 to Cooper et al, wherein paint is stripped from a relatively small object by contact with the vapors from a boiling solvent composition. In these processes the hot vapors condense to liquids on the painted surface. The resultant hot liquid not only provides a high local concentration of the paint stripping composition, but it also washes off any soluble components of the coating or breakdown products thereof.
Such methods are not applicable for removing organic coatings from extensive surfaces because of the cost of heating a sufficient amount of solvent to reflux is prohibitive and moreover, expensive equipment would be needed to carry out such an operation on a large scale. Furthermore, in some constructions, such as large metal tanks and ships, even a moderate temperature differential from one part of the construction to another can be harmful.
It is the current industrial practice to remove paint and other protective coatings from large tanks and other large constructions by the slow, unpleasant and relatively expensive procedure of abrasive blasting. It is important that a ballast tank of a ship, which usually carries ballast water, be rust-proof. To this end, ballast tanks are coated with a layer of paint. If the paint coating blisters or fails in any way, it is necessary to remove the paint from the interior of the ballast tank and repaint, to avoid the possibility of rust and eventual holes. This is especially important for ships which carry liquified natural gas. A ballast tank of a ship may have a capacity as large as one million gallons or more and often has a complex "honeycomb" configuration which makes it difficult and laborious for a blaster to work through. Also, the removal of the large amount of blasting grit needed is costly.
To date, even though abrasive-blasting has severe disadvantages, it is practically the only procedure in use for removing paint from large surfaces; hydroblasting and even pounding with hammers are sometimes employed.
There is a tremendous demand for more effective and less labor-intensive methods for cleaning fixed storage tanks, tank trucks, railroad tank cars, and barge and ship holds of residual tar, pitch, asphalt, and petroleum and vegetable oil residues of many kinds preparatory to a change in type of cargo, structural repairs, or inspection by government agencies. Some of these tanks and holds are very large, for example, 20,000,000 gallons or more in capacity.
At present they are cleaned mainly with hand-held high pressure streams of water-based solutions or emulsions, often accompanied or followed by scraping with shovels and other hand tools. In large tanks, scaffolding must be used. Labor costs, insurance charges, and long turn-around times run costs very high and the cleanliness achieved is often marginal or unacceptable, especially in the case of asphalts and other pitches, high-paraffin deposits from many crude oils, and various vegetable oil residues.